Fast, accurate and sensitive detection equipment is highly demanded in dealing with the increasing problem of worldwide smuggling, drug trafficking and acting of terrorism. At present, the ion mobility spectrometer with its lightweight, fast, and sensitive performance has been widely used in public security checking points such as in the airport and sport stadium. Ion mobility spectrometry is a relatively new gas separation and detection methods. Different from the gas chromatography, the compounds to be separated and detected in the ion mobility spectrometer must be ionized, and then under the effect of the electric field in accordance with the molecular size of the ion to be separated.
In principle, there are two categories of ion mobility spectrometers may be used in various security agencies. The first category is the traditional ion mobility spectrometer (IMS). In IMS, the operation mechanism involves application of a dc axial electric field in a tube filled with neutral gas, where ions are accelerated by the electric field and collided with neutral gas and reaches its velocity which is proportional to the field strength (v=KE).
Here v is the velocity of ion, E is electric field strength, K the ion mobility which is inversely proportional to the ion's cross-sectional, thus is a measure of ion size. The Differential Mobility Spectrometer (DMS), as the second category, makes use of the difference of ion mobility between in the high field and low field, to achieve the separation of ions. The principle is as follows: ions enter the gap between a pair of parallel electrodes and travelling along with a gas flow. One of the electrodes is applied with an asymmetrical RF waveform including a short period tH of high field EH and a longer period tL of opposite low field EL. The net drift of ion after the high and low field period is zero, that is KHEHtH+KLELtL=0, where KH and KL are the ion mobility and are assumed equal. However, it is common that ion mobility changes from a low field condition to the high field condition. This results in the net transverse displacement being non-zero. A compensation DC voltage, which is normally applied on another parallel electrode, is used to cancel the displacement, to ensure that the ions can flow through the gap between electrodes and be detected at the exit. Since difference of ion mobility in high and low field is changed from ion to ion, the required compensation voltage is changed with ions, so these different compensation voltages can be used to distinguish different ions. FAIMS, the high field asymmetric ion mobility spectrometry is just one variation of DMS and shared the same principle of operation while its electrodes use the concentric cylindrical geometry. Compared to the mass spectrometer, the resolution of the ion mobility spectrometer is relatively lower. However, because it can operate in low vacuum or even atmospheric pressure condition, thereby reduces the cost of vacuum system, the ion mobility instrument gains advantage for being made into a relatively compact instrument for quick on-site testing.
A widely used sampling method for IMS by security departments is to use paper swab to get small amount of substance from the surface to be sampled and desorb the substance from the swab in the ion source of IMS. The current limit of detection of this method is usually ng to pg levels. However, the limitations of this method is the need to have someone perform the sample collection from targeted mater or persons, and a localized sampling can not cover all of the measured object, so easy to miss the contaminated part.
To solve this problem, another security equipment with ion mobility spectrometry has been developed where the mater to be checked need to enter an air shower compartment and part of drained air is lead to the ion mobility instrument. In this method, trace amount of brush out substance is mixed with large amount of environment air, so the concentration of the sample would be much diluted thus even higher sensitivity of the detection is required. In order to further improve the instrument sensitivity, pre-concentration of sample before test is proposed. As described in U.S. Pat. No. 6,345,545, the sample particles that flow into the detector are absorbed by a series of absorbing meshes. After a period of time for absorbing, the sample is subjected a strobe of heat making online desorption from adsorbates and the gas phase of sample is introduced to the next level of detecting system. Such enrichment method has been implemented in Syagen's “Guardian Portal”. However, within this approach, there are still large number of analyte can not be absorbed and the adsorption and desorption requires a relatively long time.
Ions produced by continuous ion source may be condensed on their way leading to the detector by employing a region where the axial electric field is minimized, and a subsequential pulsed extraction may be used for sending them to the detection system. These devices have been disclosed by Anthony Jenkins as an “ion-enrich region” in U.S. Pat. No. 5,491,337, and by Richard Smith as an hourglass ion funnel described in European patent EP1678738. However, such ion enrichment devices did not separate or select the ion to be enriched, therefore it is non-selective enrichment. Above methods are neither meant to be working in high flow condition as the high flow velocity might cause instability of ion in the enrichment region, so the sample throughput is limited.
However, if ions are driven by the electric field and by the gas flow in an opposite way at the same time and two forces applied to the ions balance each other while more ions are keep coming, this species of ion can be concentrated. The concept of the balance dated back in 1898 and is proposed and implemented by Zeleny (J. Zeleny, Philos, Mag., 1898, 46, 120-154). However, the purpose of the experiment in that time is to measure the speed ratio of two ions under the application of the electric field, so no ion enrichment is detected. Later, Loscertales (J. Aerosol Sci. 1998, 29, 1117-1139.), and Tammet (Aerosol Sci. 1998, 29, S63-S64.), brought the idea of balance between electric field and flow to the differential mobility analyzer (DMA), in which a superimposed axial electric field is used to cause stagnation of ion's axial motion by the influence of flow as well as the axial field, so the ion can only move towards detector. This method only improved the resolution of DMA, but no enrichment can be achieved since the retention time in the drifting tube is very limited as the result of existence of axial field component.
Flagan (Aerosol Sci. Technol. 2004, 38, 890-899.) and Rockwood (U.S. Pat. No. 7,199,362), also used this balance mechanism in a series of ion mobility spectrometer. Different from instruments mentioned above, Flagan and Rockwood, used the air flow rather than electric field in a direction perpendicular to the balanced forces, so their method is called cross-flow ion mobility spectrometry. Similar to Loscertale and Tammet, this method didn't achieve the enrichment of the ion even though the balance of flow and electric field is satisfied, because again there exists the radial force.
Satoshi Ichimura and colleagues proposed another way of using gas flow and axial electric field for ion enrichment which is disclosed in U.S. publication No. 2003/0213903. In this method, the analyte ions going against the constant gas flow in the drift tube with diameter gradually reduced, while the electric field causing ion drift in axial direction is constant. As the diameter decreases, the ions are subjected to the increased reverse flow velocity. An ion with certain ion mobility can be stopped in a place where the ion drifting velocity caused by the electric field and flow velocity equals. As this kind of ions having net velocity approaches to zero, the continuous arrival ion can be enriched in this place. This method has the advantage of not only that the analyte can be enriched and detected, but also can filter out the small molecular ions generated in atmospheric ionization source (with greater mobility) using a weak exclusion electric field (for example in the negative mode, exclusion of oxygen ions), thereby reducing the space charge effect on ion concentration and detection. However, the way relying on changes in diameter to change the gas axial velocity ignored the gas radial velocity changes, the radial velocity component of the ion causes rapid movement to the wall, so that the ion concentration becomes very difficult. The Laiko in his article (J. Am. Soc. Mass Sepctrom. 2006, 17, 500-507) simulated the case in a similar instrument, in which ion motion affected by the flow in radial direction is just used to eject ion through the side wall of drift tube. This gives an evidence of difficulty in the enrichment method proposed by Ichimura's patent.
In another U.S. Pat. No. 7,368,709, Roger Guevremont describes the use of uniform flow, and the gradient of the axial electric field to separate ions with different ion mobility. However, this method can only be able to collect the ion group that is selected by one of the compensation voltage (CV) of the DMS. In another word, all survived and enriched ions must have single differential mobility. Large amount of useful and informative ions will be lost. Another limitation of this method is that ions concentrated in the tube are finally detected by removing them out of the tube in the axial direction, so the ions that have been already separated according to their mobility may be diffused again in the process of detection, thus the resolution of separation is deteriorated. Also, the introduction of ions from the ion source should be suspended during the detection period, therefore reduces the operation efficiency.